This invention relates generally to web transport systems and is particularly directed to a speed control for web transport systems wherein a web is advanced between a pair of rotating spools.
Microfilm readers, tape recording and playback devices and the like typically include a web transport system for advancing a continuous web between a take-up spool and a supply spool. In microfilm readers, the web is in the form of an elongated strip of microfilm, whereas in tape recording and playback devices the web is an audio or video tape. Regardless of the form which the web takes, it is a requirement of most such systems that the web be able to be transported between a take-up spool and a supply spool quickly. During such transport, it is desirable that no slack be permitted to develop in the web to ensure that the web is wound tightly on the take-up spool and no excess loops of web are formed on the supply spool.
To achieve the desired results, prior web transport systems which use a pair of drive motors, one for each spool, have included a servo control system for controlling the speed and the torque of the drive motors in order to control the tension in the web. Such control over the drive motors has been achieved by coupling each drive motor to a servo generator and using the output of the generator associated with the take-up spool to control the speed of the motors. However, when the take-up spool is rotating at low speeds, the output of its associated servo generator has a correspondingly low amplitudes, thereby making it difficult to achieve good resolution in the servo control system.
An additional difficulty associated with such prior systems derives from the fact that the speed of the web varies considerably as it is transported from the supply spool to the take-up spool. For example, if it is desired to completely transport the film from the supply spool to the take-up spool in a predetermined transport time, the initial transport of film occurs at a slow rate because of the small effective diameter of the hub on the take-up spool. As the level of web increases on the take-up spool, its effective hub diameter increases. As a result, and if the take-up spool is being rotated at a constant speed, the speed with which the web is transported increases continuously. Consequently, 70 to 80% of the transport time is devoted to advancing half the length of the web. The remainder of the web must, therefore, be transported in only 20 to 30% of the allotted transport time. During the transport of the remaining half of the web, the web is advanced at a very high speed and the supply spool rotates at a correspondingly high speed.
Several undesirable results are obtained from such operation. Firstly, it becomes very difficult to locate a portion of the web which is being transported at a very high speed during the last 20 to 30% of the transport time. This is particularly undesirable in microfilm readers. Secondly, the high speed of rotation of the supply spool, as well as the high speed of transport of the last half of the web, poses a potential safety hazard because the edges of the web and the edges of the supply spool may inflict injury to an operator who comes in contact with them. In addition, high speed rotation of the supply spool frequently results in a great amount of wear on the rotating elements and an undesirably short life span for those elements. These and other problems associated with prior web transport systems have rendered them unsuitable in some environments.
Accordingly, it is an object of this invention to provide an improved speed control for a web transport system.
It is a more specific object of this invention to provide a control for a web transport system which causes the web to be transported between a supply spool and a take-up spool at a relatively low maximum speed but within the same transport time allocated for prior systems.
It is a further object of the invention to provide a control for a web transport system which causes the first and second half of the web to be transported between spools in substantially equal time intervals.